With the spread of satellite broadcasting, satellite communication, high-vision telecasting, portable telephones and the like, transmission and reception of high-density information by a radio wave are widely conducted and the frequency of the radio wave used is being increased. With reference to the definition of the term "high frequency", conventionally it means frequency of higher than 3 MHz of HF band which is short wave while it gradually changes to mean the higher frequency such as frequency of higher than 30 MHz of VHF band which is ultrashort wave, that of higher than 300 MHz of UHF band which is microwave and furthermore that of higher than 1-3 GHz which is quasi-microwave band. Thus, the term "high frequency" is not necessarily clear in the frequency meant by it.
In any of the fields of high frequency, the materials to be used are preferably those which are small in dielectric constant and dielectric loss tangent, especially small in the latter in order to reduce transmission loss. If these are great, a part of energy given as high frequency causes intermolecular friction in the materials to lose it as heat. Resins which are small in dielectric constant and dielectric loss tangent include polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkylvinyl ether copolymer resins, polymethylpentene and the like, and these resins are used for high frequency band. Furthermore, recently it is proposed to use thermoplastic norbornene resins.
However, even in the same high frequency fields, problems sometimes occur due to purpose of use and frequency used. Especially, in the high frequency of higher than 1 GHz, when connection is carried out with a connector comprising an insulator made of a resin conventionally used for high frequency band, there is a problem of decrease in output owing to the increase in reflection of input energy at the connecting portions.
As indications which show magnitude of reflection in the resulting insulators, there are voltage and standing wave ratio (VSWR), return loss value (dB) and the like. With the smaller voltage and standing wave ratio and the greater return loss value, the reflection of energy is smaller and the better insulators are obtained. For example, in the case of the frequency of higher than 1 GHz, materials which are considered to be able to be actually used are those of 1.20 or less in voltage and standing wave ratio.
Hitherto, insulators for connectors which are prepared by molding tetrafluoroethylene-perfluoroalkylvinyl ether copolymer resins or polymethylpentene have been practically used. These resins can be subjected to injection molding and have a small dielectric loss tangent of 0.0003 or less and a small dielectric constant of 2.20 or less in the range of 1 MHz-10 GHz. However, at a frequency of 1 GHz or higher, it is difficult to obtain a voltage and standing wave ratio of 1.40 or less in insulators produced from these resins.
Furthermore, in the case of insulators comprising polytetrafluoroethylene which are put to practical use, the dielectric loss tangent is small, namely, 0.0004 or less and further the dielectric constant is also small, namely, 2.10 or less in the range of 1 MHz-10 GHz. Furthermore, insulators having a voltage and standing wave ratio of 1.20 or less at the above frequency can be obtained from the resin. However, this resin cannot be injection molded and is shaped by cutting, and, hence, the problem is that mass-production is difficult.
Moreover, even the insulators made of the same material, upper limit of the usable frequency varies depending on the structure. The smaller insulators can use the higher frequency. Furthermore, when a space is provided in the insulator so as to allow the air having a dielectric constant of 1 to be present and this space is made larger, it can be used at a higher frequency. However, when the space is provided in especially a small insulator, there occurs a problem in strength.
On the other hand, thermoplastic norbornene resins can be injection molded and have a small dielectric loss tangent of 0.0004 or less and a small dielectric constant of 2.25 or less at 1 MHz-10 GHz. However, they have never been actually used as insulators for connectors, and there have been known no examples where the voltage and standing wave ratio was measured at any frequency. Thus, it is utterly impossible to forecast what degree of voltage and standing wave ratio can be obtained in insulators for high frequency band which are made of the thermoplastic norbornene resins.